I co-authored "Truth Tables And Inference Rules" along with Hugues Leblanc
and George Weaver, Reports On Mathematical Logic, 1977.
Fortuitously Weaver co-authored a paper with Ralph Mckenzie, who co-authored a paper with Saharon Shelah, who co-authored a paper with Paul Erdos. This gives me an Erdos number of 4! That's four, not four factorial.